Technical Field
This disclosure relates to cardiopulmonary instruments and more particularly to methods and devices for automatic cardiopulmonary resuscitation (CPR) systems.
Description of the Related Art
Mechanical cardiopulmonary resuscitation (CPR) devices have switched from pneumatic devices to electro-mechanically powered compression mechanisms. These devices use battery powered motors and provide precise control and adjustability of compression rate and depth. The American Heart Association (AHA) and the European Resuscitation Council (ERC) have each established best-practice guidelines for cardiopulmonary resuscitation. A key variable specified by both is the depth of compressions. To reach the specified depth of chest compression, a significant amount of force is required. In addition, the amount of force required to reach the specified depth varies from patient to patient.
Mechanical CPR devices currently on the market may be divided into two configurations. These include rigid structure devices and chest-mounted devices.
For rigid structure devices, the chest compression mechanism or “chest compressor” is suspended above the patient using a rigid frame or structure, with a rigid backboard underneath the patient. These devices usually have some means of height adjustment to accommodate patients of different sizes. In rigid structure devices, although the chest compressor's stroke distance with respect to the chest compressor's housing can be tightly controlled by the drive mechanism's control system, the actual compression depth applied to the patient can be substantially less than the therapy depth range specified in the AHA and ERC guidelines. A significant portion of this error in patient compression depth can be attributable to the mechanical deflection of the support structure and backboard, due to the high compression forces required for adequate perfusion. This inadequate compression depth may result in adverse patient outcomes.
For chest-mounted devices, the chest compression mechanism or “chest compressor” is directly supported by and rests atop the patient's chest, and flexible straps and/or belts (hereafter “straps”) are used to affix the compression device to the patient. These devices may or may not utilize a rigid backboard underneath the patient. In chest-mounted devices, although the chest compressor's stroke distance with respect to the chest compressor's housing can be tightly controlled by the drive mechanism's control system, there are a number of reasons why the chest compressor may lift up off the patient's chest. When this happens, the actual compression depth received by the patient can be substantially less than the intended compression depth. Inadequate compression depth may result in adverse patient outcomes.
There are a number of potential sources of lift-off. It is up to the caregiver to sufficiently tighten the straps to ensure that the chest compressor is properly secured. If not done correctly, this could result in chest compressor lift-off. The chest compressor may also lift up off the patient's chest due to physiological changes in the patient's chest during therapy, including, but not limited to a change in the cross-sectional shape (e.g., “chest molding” or “compression set”) and/or changes in the relative stiffness of the chest. In addition, while the compressor's stroke may be the primary source of these changes, the straps themselves, due their shape, position, and tension, may also contribute to this effect. In these scenarios, while the straps and chest compressor may have been secure at the start of therapy, they may begin to loosen, and the chest compressor may begin to lift off the chest as compression therapy progresses. During compressions, the straps will carry a tensile load corresponding to the compression load applied to the patient. The tensile stiffness of the straps, a function of the straps' material and construction, will determine how much the straps stretch during compressions. This stretch will also contribute to the chest compressor lifting off the patient's chest.